Repair of storage tanks can be a major expense for oil, gas and petrochemical facilities.
Corrosion can lead to leaks, environmental damage and even the shutdown of operations, costing millions of dollars a day. Identifying products and processes that reduce downtime for repairs and extend the time between repairs is critical.
Industry standards and recommended practices highlight biggest challenges
API Recommended Practice 652 provides guidance on achieving effective corrosion control in above-ground storage tanks by application of tank bottom linings. Tank bottom linings are the key to preventing internal corrosion of steel tank bottoms because when stored, oil and water separate due to gravity. Water falls to the bottom due to its heavier weight and then acts as a corrosive because of its electrolytic properties. To minimize this corrosion and the downtime it can cause, industry has focused on improvements in tank bottom lining materials, surface preparation, lining application and cure speeds.
Lining innovation driven by storage tank design
Storage tanks come in a variety of sizes, with diameters sometimes exceeding 250 feet. As tanks have gotten bigger with increasing demand for storage and processing, the properties of the floor linings — and their ability to perform under greater commodity loads — have become critical.
The design of storage tank bottoms presents special challenges to linings. Tank floors are typically lap plate construction, consisting of a set of steel plates joined together using lap welds. Linings must be able to adhere and provide adequate edge retention at these weld joints. Floating roof tanks, which feature support legs and welded-in-place contact striker plates on the tank floor, also offer tests which the lining must address.
Finally, effective linings must take account of the way repairs are made to storage tank bottoms. When tank floors are compromised, plate steel is usually scabbed over holes or other severely pitted surfaces, creating an irregular surface.
To handle all these challenges, lining manufacturers have focused on 100% solids, high-build solutions with greater film thickness, excellent flexural modulus and elongation characteristics, and edge retention properties. Some are strengthened with ceramic beads, glass flakes or milled fiber reinforcements to provide even greater durability and resilience.
There are several variables that owners should consider when choosing a Maintenance, Repair and Operations lining for their storage tanks.
- Ease and speed of use: The ability to spray at high temperature and pressure and achieve high film build quickly is critical to fast turnaround times. This is the key benefit of newer 100% solids solutions, which leverage plural component spray systems to reduce time and labor costs, versus solvent-based, multi-coat systems. These 100% solids systems have also shown to last up to two times longer.
- Geographical location and time of year: Solutions that work well for tanks in Calgary may not be optimal for those located in Texas. A low-temperature-cure solution, for example, is probably not the best for repairs done in high heat.
- Stored temperature of the commodity: The temperature of the commodity being introduced into the tank will also impact the appropriate lining solution. If the commodity needs to be heated, it may cause thermal shock to a tank in a cold location, necessitating a specific kind of lining that can resist thermal shock and elevated temperatures.
- Purity of the commodity: Another factor is the type of commodity being stored, such as traditional unleaded vs. ethanol-based gas, or commodities with colorants added. Additives can attack linings in various ways causing them to leach into and contaminate the commodity.
- One system for both pinhole and major repairs: Most contractors doing storage tank maintenance will do a spark or holiday test to check for pinholes or voids at the same time they are checking for more significant repairs. Having a lining system that can be applied in both a hot pot/single leg, brush and roll process as well as via a plural component spray system will make the contractor’s job easier and quicker.
Many coatings manufacturers offer a range of 100% solids, high-build systems for tank bottom repair. What differentiates them is their level of knowledge and experience with specific commodities and locations. Field service capabilities and a network of trained contractors also play a critical role in ensuring tank maintenance goes as smoothly as possible.
With the critical nature of petroleum tank linings, owners often desire to see test results for the specified internal linings in an autoclave at specific temperatures, pressures and commodities for added piece of mind.
For more information, visit ppg.com.
